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How does speaker quantity influence port tuning?

Discussion in 'Amps and Cabs [BG]' started by patrickj, Sep 16, 2002.


  1. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    I've read through the entire 'Speaker Building 101' (thread from mid-2001) but couldn't really find a answer clear on this.

    Port tuning for a single speaker is relatively easy given Vb, Fb, and the T/S values, etc. How does a multiple speaker cabinet affect port tuning though? (assertion: all speakers are the same size)

    For example (just throwing out #s), suppose a 10" driver requires (2) 2.75" ports @ 3" length. Would two 10" drivers require (4) ports, three would require (6) ports, etc?

    My instincts say that tuning is on a per driver basis, so combine the port area of a single driver w/ the number of drivers, then work with this result to get workable ports. Can anyone clarify this for me?
     
  2. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    No, it's not that simple because port dimensions also depend on box volume.
    For simple online calculation see http://www.linearteam.dk/ventcalc.html
    At this website you can also download WinISD, a very useful program for cab design.

    Matthias
     
  3. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    To be more precise:
    Port dimensions depend on box volume (Vb) and tuning frequency (Fb), and not directly on driver parameters.
    However the driver parameters dictate Vb and Fb within certain limits if you want to achieve useful results.

    Say your driver requires a 100 litre box tuned at 40Hz. To achieve this, you can use (2) 2.5" ports @2.846" length.

    If you want to use 2 of those drivers, you would need a 200 litre box, also tuned at 40Hz. And in this case, yes, you can use (4) 2.5" ports @2.846" length. But you could also use (2) 2.5" ports @0.508" length or any other configuration which makes sense.

    Bottom line: The number of drivers affects the required box volume. Once you know this and Fb, the number of drivers is irrelevant for calculation of port size. (Well, actually you also have to take into account that 2 drivers have twice the acoustic power and this increases vent mach...)

    Hope this helps,
    Matthias
     
  4. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    Helps a ton.. thanks!

    ISDOnline is what I've been using for calculating everything (Vb, ports, etc). I initially thought that port size/tuning was directly dependent on Fb & Vb, not the # of drivers - but then I saw a few posts in the other topic that made things less clear.

    Thanks again!
     
  5. Port dimensions are also directly tied to the driver parameters.

    The cross sectional area of the port is dependent on the amount of volume displaced by the cone at maximum power. A driver with an Xmax of 0.1" requires less port area than a driver with an Xmax of 0.5".

    The port tunes the internal volume of the cabinet to a specific frequency. As long as the internal volume is constant, a port of given size will tune to a given frequency. If you add more drivers inside that cabinet, the net volume is reduced, and the port is effectively larger, relative to the smaller volume. This changes the tuning.

    The real issue with multiple drivers is how it affects the Vas elements. When you increase the driver count from 1 to 2, you also have to double the box volume. This requires a change in the port to maintain the same tuning frequency. Multiple drivers move more air, so the port must be enlarged accordingly.
     
  6. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    Do you mean a small reduction in internal liter/cubic feet.. equating to the actual space the drivers take up, etc (also take into account all bracing, etc)?

    I've accounted for the bracing (dimensions of rectangles, easy enough to figure out), but for the actual drivers I guestimated using volume of a cone/per driver. It's not 100%, but it's close I think.

    I used WinISD (online - I'm on a Mac) to calculate everything. When calculating box volume I specified how many drivers for the box, and it gave me the optimum cubic feet/liters for that quantity of drivers. Using that Vb (and corresponding proper Fb), that will ensure I have correct port tuning. (I built slightly larger taking into account the loss of internal volume due to drivers, bracing, etc)

    I'm lost on the 'Multiple drivers move more air, so the port must be enlarged accordingly'. Using what Mathias stated above about combining acoustic power, I found that for the total # of drivers (4), the vent quantity/length/dimensions remain unchanged (quantity 3, 2 7/8" dia, 4.3" length). The vent mach did increase by .01 (.01 -> .02 - ISD says stay below .16).
     
  7. I'd suggest you download my spreadsheet... but you can't because you're a Mac user.

    Sorry

    :D

    If you post the exact model of driver you are using, I can give you the data, if I have it.
     
  8. BenF

    BenF

    Mar 29, 2001
    Boston area
    Your spreadsheet works fine on my mac, Patrick shouldn't have a problem.
     
  9. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    The spreadsheet works fine (Excel for OSX). The drivers are Madison's NWX, which aren't included in the Ref. I've already got the T/S parameters (if that's what you mean) which I used in order to calculate optimum Vb, Fb, ports, acoustic power, etc.

    I haven't been able to find multiple speaker construction examples that were ported. They were either single speaker cabinets/speakers, or single cabinets with multiple sized speakers all seperately enclosed, etc, or there's not enough elaboration with multiple ports.


    http://www.linearteam.dk/ventcalc.html

    For example.. (hypothetical #s)

    a 2x15 cabinet needs Vb = 8.5 cu/ft, Fb = 29.97.

    Start with (1) vent @ 5" dia (straight, not inner flanged).. ISD suggests the vent be ~ 3.3" in length. Vent mach is ~ .03.

    My question is basically.. is this <i>per</i> driver? Or is this for the overall cabinet irregardless of how many drivers are installed (as long as Vb matches). The program doesn't ask about multiple drivers, and as multiple drivers are moving more air, is there anything that needs to be done to compensate?

    As a former wind instrument (trombone) player, I know that more air through the horn doesn't equal a change in pitch - sort of the same principal here - but I just want to verify from someone with more experience than I have.
     
  10. Link to Port Calculator

    Please email me all the T/S data you have, and I will add it to my spread sheet.

    The number of drivers determines the amount of port area. The number of ports is a sum of the required area (kinda).

    JL Audio proved it and the full explanation is on their site, but you can divide the cabinet volume by the number of intended ports (8.5 cubic feet / 2 ports = 4.25 cubic feet per port). You can plug these numbers into WinISD, etc to find the proper diameter and length.

    The only advantage to using multiple ports is that smaller diameter pipe is easier to find and less costly to buy.

    If using more than 1 driver, the cabinet volume is the multiple of the volume required for a single driver. One driver Vb = 4.25 cubic feet, two drivers = 8.50 cubic feet.

    I would heartily encourage you to build two 1x15 cabs. They are MUCH easier to haul around than a 2x15 is. The smaller cabs also have less tendency to buzz because the panels are smaller. Then you can use 1 driver and 1 port, and avoid confusion.
     
  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    Here's how I look at it:

    1. start with driver parameters

    2. find a suitable alignment for a single driver (giving you Vb and Fb)

    3. How many drivers total? Multiply Vb by this much

    4. Fb stays the same, so start calculating port parameters for the new Vb and the same Fb

    5. Pick a set of (port area,port length) where the area is great enough to handle the air from the total driver area you're using.

    That's for the whole box, at least as far as diameter and length goes.

    The "Mach" calculation is just done on a total sound pressure level output, I think. It's possible that it is dependent on the area of your driver(s). But generally, if a 5" diameter port is sufficient for one driver, two of them (or some other configuration of equal area) should be sufficient for two drivers. Of course, since the box size is twice as large the length will be different.
     
  12. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    Arigato all!


    bgavin, emailed you the url..


    *edit*

    that port calculator is MUCH more informative/useful than the ISDonline calc..
    thanks!!

    also, I'm not building a 2x15.. just an example that I figured would be easy to demonstrate.
     
  13. I didn't get the email, please post here.

    thanks
     
  14. Which driver are you contemplating?
     
  15. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    the 1590

    vas: 11.37
    qts: .44
    fs: 27

    vb: 5.5
    fb: 10

    [​IMG]

    suggestions?

    this is more of an experiment than anything else, if it works - great - and i'll start another (higher Qualtiy components).
     
  16. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    with an Fb of 25, there's a slight boom, but the 30hz is a little higher in db..

    [​IMG]
     
  17. I can't find either the Sd or Xmax values. Do you have more info on the 1590 driver?
     
  18. patrickj

    patrickj

    Aug 13, 2001
    Baltimore, MD
    That's all I've got.. (everything on the page), etc.
     
  19. That is, if the port fits into the box :). 25 Hz is pretty low, and requires a pretty long duct. Besides, you have to account for the volume of a port that large/long. Subtract it from the total cab volume.